Spasmodic dysphonia (SD) is a disorder in which one or more laryngeal muscles spasm during phonation. It was first described in 1871, and the modern description was published by Aronson in 1964. SD manifests as a focal laryngeal dystonia affecting the control of vocal fold movements during voicing. It is a chronic, focal, movement induced, action-specific dystonia of the laryngeal musculature during phonation. The condition makes speech very difficult for those affected, and consequently it is an isolating and even debilitating problem. It can have a profound effect on quality of life, severely limiting people's communication, especially via telephone, during public speaking and in noisy backgrounds.
Two subtypes of this disorder have been identified. The first is adductor spasmodic dysphonia (ADSD), characterized by breaks in vowels attributable to thyroarytenoid (TA) muscle spasms resulting in intermittent hyperadduction of the vocal folds. The second subtype is abductor spasmodic dysphonia (ABSD), characterized by intermittent breathy voice and sudden pitch breaks. In ABSD, symptoms are thought to be due to spasms of the posterior cricoarytenoid muscles and, in some cases, the cricothyroid muscles. The incidence of prolonged vocal breathiness after voiceless consonants is thought to occur as a result of sustained vocal fold abduction due to the dystonic muscle spasm. ABSD is quite rare.
The characteristic vocal breaks of Adductor SD are due to spasmodic hyperadduction of the vocal folds that interrupt phonation. As a result of the force of the spasms, which generally occur with voiced vowels, vocal fold closure interrupts phonation causing a strained or strangled vocal quality with intermittent vocal breaks.
Historically treatment of SD has been aimed at paralyzing or weakening one of the vocal folds in order to decrease its ability spasm and interrupt phonation. Dedo first proposed and popularized recurrent laryngeal nerve resection for SD. This was the first and only surgical procedure which achieved widespread use. Some surgeons did not want to completely transect the RLN, and instead crushed it to weaken the vocal folds but keep the nerve intact. Unfortunately, over the long term, the majority of patients who underwent either resection or crush experienced a return of their phonatory spasms. Due to this, both procedures were eventually abandoned.
A number of other procedures designed to decrease the strength of the vocal fold contraction have been tried. Isshiki proposed the type 2 laryngoplasty for adductor SD. This procedure relaxes and lateralizes the vocal folds. This anatomic alteration prevents the vocal fold spasm from forcefully obstructing airflow and causing a vocal break. If this procedure is overdone, a patient's speaking voice can be excessively raspy and breathy. In his series of 41 patients, 70% patients reported excellent results after six months. However, a number of other surgeons have not been able to replicate Isshiki's success.
Despite the surgical failure of thryoplasty others have attempted surgery to weaken the vocal folds to treat SD. Koufman described bilateral thyroarytenoid and lateral cricoarytenoid muscle resection. Short-term results in 5 patients revealed improved fluency in all patients, but long-term studies are needed, especially considering the failed history of performing myectomy for other dystonias, such as blepharospasm, which not only fails to prevent recurrence of symptoms but often causes the muscle to become dysfunctional due to fibrosis and scarring.
Sercarz et al performed research to develop a surgery that would result in a similar physiologic outcome of vocal fold weakening with improvement of SD. They first performed selective recurrent laryngeal nerve (RLN) adductor branch denervation of the adductor muscles in the canine. They confirmed this surgery would decrease a patient's ability to forcefully adduct the vocal folds by showing an inability for the development of high levels of subglottic pressure despite maximal RLN stimulation after RLN denervation. They then performed selective laryngeal adductor denervation-reinnervation in humans in 1999. In this procedure, the adductor branch of the recurrent laryngeal nerve is transected bilaterally. The distal branch is then reinnervated with the ansa cervicalis to provide vocal fold tone and prevent reinnervation by nerves affected by SD. In addition, to further weaken the vocal folds a lateral cricoarytenoid myectomy is performed. A retrospective study by Chhetri et al showed long-term (mean 49 months) improvement in both patient subjective voice evaluation and expert perceptual voice evaluation. 26% of patients had continued voice breaks, and postoperative breathiness was significant in 30%. This surgery has not been accepted and is primarily performed at the institution in which it was developed.
The injection of botulinum toxin (BTX) into the larynx is the treatment of choice for adductor SD. It is believed that the BTX decreases the motor activity of the recurrent laryngeal nerve (RLN), inducing paresis of the vocal folds and therefore preventing phonatory interruption with vocal spasms, essentially performing a chemical neurectomy which denervates and weakens the muscles. Botulinum toxin is produced by Clostridium botulinum. The thyroarytenoid muscles are usually injected with BTX under electromyography (EMG) guidance via a cricothyroid membrane submucosal approach. It causes a chemical denervation by splicing the fusion proteins (SNAP-25, syntaxin or synaptobrevin) and blocking the release of acetylcholine at the synaptic junction. Botulinum toxin A and B are used in humans with A being much more common. The clinical effect of botulinum toxin in SD is classically thought to result from reducing the activity of the motor nerves, thereby weakening the laryngeal muscle and weakening its spasms.
Patients with SD who have had laryngeal injections of BTX experience smoother speech with fewer voice breaks. However, the injections do not completely control symptoms. The injections must be repeated approximately every three months. They are painful and uncomfortable. A relatively small number of medical practitioners perform the injections, so access to the treatment can be difficult. Immediately following an injection patients experience side effects such as a weak voice. Prior to the following injection patients' spasms return. As such, there is a sinusoidal symptom curve, and symptoms are optimally controlled for only a portion of the treatment cycle.
Oral medications used for dystonia such as anticholinergics, benzodiazepines and baclofen provide minimal relief and have high side effect profiles at the doses needed to effect the voice.
Voice therapy is of minimal benefit for the symptoms of SD. It is helpful in resolving the poor compensatory behaviors patients have developed in attempts to decrease their spasms. Often these behaviors spontaneously resolve after injections are begun but that is not always the case. There is a small amount of evidence that voice therapy may augment the effects of BTX.
Most applications of neuromuscular stimulation in the larynx have been aimed at reanimating paralyzed muscle to relieve laryngeal paralysis or paresis rather than reducing muscle spasms to treat SD. Neuromuscular stimulation intended to treat laryngeal paralysis acts on motor nerves to cause muscle contractions. Neuromuscular stimulation as a treatment for SD has been limited to a single study exploring the use of efferent stimulation intended to act on motor nerves and impact the nature of muscle contractions. Bidus, et al attempted to determine whether efferent stimulation of the adductor muscle would have potential as a treatment alternative for patients with abductor spasmodic dysphonia (ABSD). Bidus stimulated motor neurons of the thyroarytenoid or lateral cricoarytenoid muscle in order to cause antagonisticadductor muscle contraction. This study involved the temporary placement of hooked wire electrodes in the thyroarytenoid or the lateral cricoarytenoid muscles in an outpatient setting. The researchers determined a stimulation threshold by observing the lowest level of stimulation which produced observable movement in the vocal folds when observed through a laryngoscope.
While this single study by Bidus attempted to treat SD through efferent neuromuscular stimulation, apart from the inventor of the methods disclosed herein, no researcher has proposed treatment of SD by specifically manipulating the afferent system or gamma motor neuron; much less, manipulating it through electrical stimulation. In contrast, SD experts predicting the future of treatment and research directions for SD point to identifying and treating the mutant CNS neural proteins that cause SD. They do not mention the treatment of SD through alteration of the afferent system or gamma motor neuron.